1. Field of the Invention
The present invention relates to a back light unit for liquid crystal display device, more particularly to a back light unit having a light guiding plate substituting for a mold frame.
2. Description of the Related Art
Liquid crystal display (LCD), as the visual graphic display devices, involves a small device size and lower power consumption comparable to other display devices such as CRT. The LCD requires a light source in addition to its liquid crystal screen since the LCD is not a self-luminous device.
Currently, lamp such as CCLF and HCFL has been used as a light source for LCD. The back light unit can be divided into the lower part lighting type and the edge lighting type according to location of the lamp. In the lower part lighting type, a lamp light is uniformed by a diffusing sheet and then emitted into liquid crystal panel. On the other hand, A lamp light of the edge lighting type becomes at first a sheet-type light source through a light guiding plate.
FIG. 1 is a drawing for illustrating a conventional edge--light type back light unit. Referring to FIG. 1, a lamp 1 as a light source is surrounded by a lamp reflector 2. A light guiding plate 3 in which a reflected light is scattered and uniformed, is disposed on one side of the lamp 1. Optical sheets consisting of a diffusing sheet 5 and a lower prism sheet 6 and an upper prism sheet 7 are disposed on the upper plane of the light guiding plate 2 and a reflecting sheet is disposed on the lower plane of the light guiding plate 3.
The diffusing sheet 5 is arranged for improving the uniformity of incident light on a liquid crystal panel (not shown). The reflecting sheet 4 is arranged for avoiding the light leakage toward the lower of the light guiding plate 3 and reflecting the light perpendicularly to the upper of the light guiding plate 3. The lower and upper prism sheets 6 and 7 are arranged for converting the light travel path and are comprised of a sequence of troughs in the shape of triangle or hemisphere. To protect the shape of the lower and upper prism sheet 6 and 7 a protecting sheet 8 is disposed on the upper prism sheet 7. The liquid crystal panel is disposed on the protecting sheet 8.
Every constituents of the above back light unit are to be fixed on a mold frame 9 which acts as a supporting structure and is formed by the injection molding method. There are four guiding ribs(not shown) acting as assembly bases for the above sheets on the corners of the mold frame 9. Each sheet is inserted within the guiding ribs and then attached to the mold frame 9 by an adhesive tape. Also, the mold frame 9 has mounting holes(not shown) for fixing the entire liquid crystal module into a monitor frame.
In the back light units as described above, the light emitted from the lamp 1 is reflected by the lamp reflector 2, and then images on the display screen is displayed by the lights passing through the light guiding plate 3 and the diffusing sheet 5. Especially, the light emitted from the lamp 1 is prevented from external leakage by the reflecting sheet 4 and then is irradiated toward the light guiding plate 3. Then the light becomes uniform by the diffusing sheet 5 and is directed toward perpendicularly to the liquid crystal panel with converting the light travel path with a selected angle while passing the lower and the upper prism sheets 6 and 7.
As back light units become thinner and lighter, there have been proposed various studies for reducing the thickness of liquid crystal module. However among those back light unit components, the thicknesses of optical sheets and light guiding plate affect the device's optical capability, thus it may impossible to reduce below certain limit. Consequently, there have been proposed various method for reducing thickness of mold frame which has no relation with device's optical capability. If the thickness of mold frame is reduced below certain limit, the injection molding is very difficult to perform and the supporting function thereof is deceased.